As a plating system in an electrogalvanizing line, there are two systems, one of which being a simultaneous both-side plating system wherein both sides of steel strip are simultaneously electrogalvanized, and the other of which being a both-side separate plating system composed of two-stage plating wherein one side of steel strip is electrogalvanized in a first plating stage and thereafter the opposed side of steel strip is electrogalvanized in a second plating stage.
The both-side separate plating system has some merits that the changeover between both-side coating and one-side coating can be achieved by merely turning over the passing direction of the steel strip to be coated, and the replacement of used electrode with new electrode can be performed simply and hence the workability is excellent.
As the both-side separate plating system, there are a horizontal type as shown in FIG. 1 and a radial type as shown in FIG. 2. In any case, a wetting tank 3 is arranged between a first plating cell 1 and a second plating cell 2. A steel strip 4 is passed through the first and second plating cells 1, 2 by means of conductor rolls 5, during which it is subjected to a galvanizing with an electrolyte 7 through an anode 6 arranged opposite to the steel strip 4.
When the steel strip 4 moves from the first plating cell 1 to the second plating cell 2, it is wetted with a wetting solution 8 in the wetting tank 3. This wetting treatment is carried out in order to introduce the steel strip 4 into the second plating cell 2 at a uniformly wetted condition of the non-plated surface, because when the non-plated surface of the steel strip 4 to be introduced into the second plating cell 2 is completely dry or is locally adhered with the electrolyte, it is apt to produce uneven plating by the second plating treatment.
Heretofore, a sulfate bath consisting essentially of zinc sulfate has mainly been used as an acidic electrolyte for the electrogalvanizing. Lately, a chloride bath becomes frequently used instead of the sulfate bath because of the following merits as compared with the sulfate bath:
(1) Since the electric conductivity is high, the required voltage can be reduced; and PA1 (2) Since the acceptable current density is high, the high-speed plating is easy. PA1 (a) Plating bath: composition ZnCl.sub.2 =200 g/l, KCl=350 g/l, temperature 55.degree. C., pH=5.0 PA1 (b) Line speed: 50 m/min PA1 (c) Current density: 50 A/dm.sup.2 PA1 (d) Plating apparatus: radial type both-side separate plating system first plating cell (bottom surface plating).fwdarw.wetting tank.fwdarw.second plating cell (top surface plating) PA1 (e) Wetting solution: composition ZnCl.sub.2, temperature 40.degree. C., pH=4.5
Incidentally, when the sulfate bath consisting mainly of zinc sulfate is used as the electrolyte, no care must be particularly taken on the wetting solution in the wetting tank 3. That is, good both-side galvanized steel strip can be obtained by using the electrolyte in itself as the wetting solution.
However, when the chloride bath consisting mainly of zinc chloride is used as the electrolyte, if such an electrolyte in itself is used as the wetting solution in the wetting tank 3, it is confirmed that the zinc coating coverage is extremely lowered in a surface to be plated in the second plating cell 2.